This invention relates to an ink-jet printer which uses an ink containing fine solid particles of a pigment suspended in a carrier liquid. More particularly, the ink-jet printer is of the type utilizing electrophoresis of the pigment particles in the ink in an ink chamber of the print head for concentrating the particles in the vicinity of an ink ejection orifice provided at an end of the ink chamber.
In known ink-jet printers of the above-mentioned type, the ink chamber in the print head is provided with a first electrode to which a steady DC voltage is applied to produce an electric field in the ink chamber thereby to induce electrophoresis of the electrically charged pigment particles in the ink toward the ink ejection orifice. As the pigment particles migrate toward the orifice at a definite rate, the particles concentrate in the vicinity of the orifice. A second electrode is disposed in the ink chamber close to the orifice. After concentrating the pigment particles in the vicinity of the orifice, a DC voltage in pulse form is applied to the second electrode to cause ejection of an agglomeration of the pigment particles together with a small amount of the carrier liquid from the orifice toward a recording surface. On the recording surface the agglomeration of pigment particles forms a single dot. By repeating this process while the ink chamber is replenished with the ink, an image is printed on the recording surface. When the pulse duration of the voltage pulse is relatively long, each pulse causes ejection of a few or several agglomerations of pigment particles one after another at nearly constant time intervals, and on the recording surface these agglomerations form a single dot of a relatively large size.
In the operation of the ink-jet printer described above, concentration of the pigment particles in the vicinity of the ink ejection orifice reaches an excessive extent if the application of a voltage pulse to the second electrode is interrupted for a relatively long period of time. Then, it is likely that the orifice is clogged with the pigment particles. Even though the orifice is not clogged, the ejection of an agglomeration of pigment particles will become unstable. These phenomena lead to degradation of the printing quality.
When the time interval between two pulses of the voltage applied to the second electrode is relatively long, there arises another problem that the ejection of an agglomeration of pigment particles by the later pulse is liable to be delayed or missed. This is because the pigment particles tend to move away from the tip part of the second electrode before the application of the later pulse of voltage to the second electrode.